Aluminium(iii) porphyrin based axial-bonding type dyads containing thiaporphyrins and expanded thiaporphyrins as axial ligands

Abstract

Four axial-bonding type Al(III) porphyrin based dyads, containing thiaporphyrins with N₃S 1 and N₂S₂2 cores and expanded thaiporphyrins with N₂S₃3 and N₂S₄4 cores, were synthesised by treating [(TPP)Al^IIIOH] with the corresponding mono-functionalised meso-4-hydroxyphenyl thiaporphyrin or expanded thiaporphyrin building blocks in benzene at refluxing temperature. The stable dyads 1–4 are freely soluble in common organic solvents and characterized by mass, 1D and 2D NMR, absorption, electrochemical and fluorescence techniques. 1D and 2D NMR studies unambiguously confirmed the dyads. The absorption and electrochemical studies show weak interaction between the two macrocycles in dyads and the constituted macrocycles in dyads mostly retain their characteristic individual features. The steady state fluorescence studies indicate a decrease in the quantum yield of Al(III) porphyrin unit in dyads 1–4 and invoked the possibility of singlet–singlet energy transfer from the Al(III) porphyrin unit to the N₃S porphyrin unit in dyad 1 and N₂S₂ porphyrin unit in dyad 2. Structural information of these dyads in their ground (S₀) state are generated by applying density functional theory (DFT) based quantum chemical calculations. Excited state calculations are carried out following time dependent density functional theory (TD-DFT) and UV-Vis spectra of these dyads are simulated in the Soret band region.